Excavators (i.e., draglines or rope shovels) are used to move relatively large amounts of overburden or ore, typically required in surface mining operations. An excavator includes a bucket, a boom, a revolving frame, and a base. An operator controlling a dragline manipulates the dragline to fill the bucket. The bucket is lifted such that it is suspended from the boom. The operator then causes the revolving frame of the dragline to turn or swing relative to the base, and dumps the contents of the bucket.
A swing drive system of the dragline is responsive to input from the operator for turning the revolving frame of the dragline relative to the base. The swing drive system includes a number of generators, electric motors, gear sets, and shafts. The generators power the motors from a main power supply, and a shaft transfers torque from each motor to an associated gear set. The shafts experience torsional stresses and may experience torsional oscillations which can cause premature failure of the shaft, the driven gear set, and any couplings (e.g., intermediate gear boxes) or bearings associated with this mechanical system of the swing drive system. Oscillations in the swing drive system also impact the boom (i.e., cause additional stress in the boom, particularly at the base of the boom).
Prior art swing drive systems used in excavators (i.e., draglines) such as the Bucyrus 1570 dragline include one or more sets of two generators and two motors. Two sets are shown in prior art FIGS. 1 and 2. The armatures of the two generators and two motors in each set (GEN1 and GEN2 are one set and GEN3 and GEN4 are the other set) are connected in series with one another. The fields of the two motors in each set are excited with a constant voltage source. Referring to FIG. 1, the fields of the generators in each set are excited by a common, variable direct current (DC) source so as to control the power supplied to the associated motors. This configuration of motors and generators is intended to accomplish load sharing and speed matching between the motors in each set and between the two sets to reduce torsional oscillation in the mechanical system driven by the motors.
Generators, for example, on a Bucyrus 1570 dragline are Frame MCF-866B, rated 836 kW, 1200 rpm, 475 volts and are equipped with shunt fields wound in accordance with data sheet 255H805XA, sheet 12. There are four generator field circuits, north and south forward circuits and north and south reverse circuits, each circuit having three poles. Each field pole is of 272 turns, has a resistance at 25 degrees C. of 0.295 ohms, and an inductance of 0.87 henries. In one prior art implementation, the generator field circuits are reconfigured such that only the 2 forward circuit of each generator are used as shown in FIG. 1.
The swing drive system motors, for example, on the Bucyrus 1570 dragline are MDV-822-AER, rated 1045 hp, 740 rpm, 475 volts, 1760 amperes and are equipped with shunt fields of 450 turns per pole. The rated field current delivers rated torque and speed. There are two motor field circuits in each motor drawing a total of 26.4 amperes when connected in parallel. The field circuits may be connected in series to draw 13.2 amperes at double the field voltage.
Referring to prior art FIG. 2, a prior art configuration of a swing drive system of a Bucyrus 1570 dragline is shown. Kirchoff's Law states that the sum of the voltages around an electrical circuit must equal zero. Thus, ideally a first generator armature 102 would produce positive 400 volts and an associated first motor armature 104 would produce a counter-emf of negative 400 volts. A second generator armature 106 and an associated second motor armature 108 would do likewise such that the sum of voltages around the armature loop 110 would be zero. However, in the four-machine armature loop of FIG. 2, the two motor armatures do not always produce the same counter-emf because of variations in their operation due to varying electrical impedances and changing load torques (i.e., gear engagement or cogging of the gears driven by the motor) and load speeds. For example, one motor can generate 420 volts while the other generates 380 volts and still satisfy Kirchoff's Law. Thus, speed and counter-emf can change at random and yet maintain a summation of around-the-circuit voltage at zero. Therefore, in the prior art shown in FIG. 2, a balance resistor 112 was added in the armature loop of each generator motor set in parallel with a motor of one pair and a generator of another pair to further balance the voltages between the motor and generator pairs in order to reduce mechanical stresses applied to the shafts and gear sets of the swing drive system.
In operation, the operator of the excavator selects an acceleration of the swing drive system via a master switch (not shown) by manipulating a controller, such as a masterswitch, control stick, a lever, or some other input device. In response, the regulator 114 applies power to the generator field circuits of each generator via a generator field exciter 116. One prior art method of controlling the swing drive system on the excavator assumes that the current in one armature loop 110 is the same as the current in every other armature loop and assumes that the voltage of all of the generator armatures are the same. The regulator 114 regulates the current (i.e., torque) applied to all of the generator fields as a function of the acceleration selected by the operator (i.e., operator input) and the voltage and current of a single generator armature such that the voltage limit (i.e., speed limit) of the motors is not exceeded.
Other prior art swing drive systems include multiple sets of direct current (DC) static motor armature power supplies associated and an equal number of DC motors. Other swing drive systems are powered by sets of alternating current (AC) variable frequency drives and an equal number of AC motors in which the frequency and voltage of the power from the AC variable frequency drives controls the torque output of the AC motors.